I would highly recommend you to:
Watch Streams: Watch some of the best SC2 streamers. Of course, watch the players which play your race. You'll see how to use new tricky units and which build to learn in order to build them. Take a look at Twitch SC2 and Team Liquid Stream Wall;
Play custom games with friends or vs AI: You'll be able to test new units without failing or being stressed;
Play Micro Arena - Arcade: You'll be only focused on using a bunch of units to win. You will learn, also, how to use some of the new units or just combine them with the old ones;
Play Unit Tester - Arcade: You'll be able to create any army you want and test every unit and its ability;
Play Unranked and Ranked Games: When you'll be a bit more confident, try starting to climb the ladder;
Play off-race games: Try playing with all 3 different races to understand deeply how new units work.
I hope I've been Helpful.